The present invention relates to skull pins for use in head fixation devices or cervical traction devices, and more particularly to the design of a skull pin which allows the cervical fixation/traction device to be better used in high powered, magnetic resonance imaging.
Cervical fixation and traction devices have long been used in medical treatment to hold a patient's head and neck in a particular stationary or immobile position. In particular, injuries to the cervical spine are typically treated by immobilization of the head and neck using some type of external cervical fixation device. Cervical traction devices typically involve relative immobilization of the head and neck followed by application of a traction force to the fixation device and therethrough to the head.
Two primary types of cervical fixation devices are "halos" and "tongs". Cervical halos have an ovular or ring-shaped support member which is circumferential or partially circumferential around the patient's head. Cervical tongs include two opposed arms which are placed for support on either side of the patient's head. Preferably, both the halo and tong devices allow the attending physician to maintain the patient's head and neck in a stationary position at any of a variety of orientations as selected by the attending physician. Either the halo or the tong may be supported from the patient's shoulders and chest with a vest and uprights. Traction forces may be applied to either the halo or more commonly the tong after placement on the patient. The assignee of the present invention provides such devices as ACE open backed halo rings, ACE closed backed halo rings, ACE-TRIPPI-WELLS tongs, ACE UNIVERSAL tongs, ACE standard tongs, ACE MARK III vests, ACE MARK IV vests and ACE halo vests.
The halo or tong is commonly attached to the patient's head with "skull pins". The skull pins have pointed tips which are directed toward bone of the patient's skull and thereby through a compression force hold the skull in a fixed position relative to the halo or tong. The skull pins are supported by and engaged in the halo or tong for controlled movement toward and away from the patient's head. Typically four or more skull pins are circumferentially spaced around the equator of the patient's head. If traction is applied, the skull pins commonly provide the traction force through a cantilevered bending force on the skull pins. The tips of the skull pins should be sterile when used so as to avoid the possibility of infection of the scalp or skin and underlying tissue at the skull pin contact locations.
Various medical imaging techniques with tomographic display have been used in diagnostic evaluation of the head and cervical spine. In the late 1980's, magnetic resonance imaging ("MRI") emerged as a promising new tomographical imaging modality for cervical spine images. MRI involves subjecting the viewed tissue to a static gradient magnetic field in the presence of second magnetic field which rotates and/or pulses with a characteristic frequency. Hydrogen nuclei or protons have a Larmour frequency in the radio frequency ("RF") range. When the second magnetic field rotates or pulses with an appropriate radio frequency, the hydrogen nuclei or protons at one plane of the static gradient magnetic field (i.e., one slice of the specimen) absorb and reemit electromagnetic radiation which can be sensed by the RF transmitting coil.
Various test sequences may be performed by the MRI device. Typical cervical spine pulse sequences include a sagittal T1 weighted spin echo series, a sagittal T2 weighted fast spin echo series, a sagittal gradient echo series, and an axial 3D gradient echo series. These sequence may be performed with and/or without magnetization transfer contrast ("MTC"), a pulse sequence used to achieve additional contrast in the image. Today, MRI is an essential technique of assessing various aspects of spinal injury and is valuable for monitoring the healing process.
A primary concern for any imaging system is the safety of the patient, and standards have been established for the amount of RF radiation in MRI which is considered "safe". Specific absorption rate ("SAR") is a dosimetric term that describes the mass normalized rate at which biological tissue is exposed to RF radiation. The SAR recommended by the United States Food and Drug Administration for the safe use of MRI systems is a whole body averaged SAR of 0.4 watts per kilogram or less averaged over the body, a SAR of 8.0 watts per kilogram or less spacial peak in any one gram of tissue, and a SAR of 3.2 watts per kilogram or less averaged over the head.
During MRI, the quality of the image generated is a second major concern. Any cervical device used in MRI must not distort the image quality generated by creating artifacts. The traditional material for cervical fixation device components, stainless steel, has produced artifacts during MRI procedures which are diagnostically unacceptable. Accordingly, titanium, aluminum, graphite, graphite composite or plastic have been used in cervical fixation devices in place of stainless steel to improve MRI image quality. For instance, skull pins today are typically made out of titanium. Other materials for skull pins have been proposed to reduce artifacts, such as a skull pin with proximal portion of boron or carbon fiber reinforced plastic and a distal portion of a single crystal alumina ceramic as disclosed in U.S. Pat. No. 4,612,930.
A third concern of cervical fixation device components is that the ferromagnetism of the components be quite small. That is, the components should not magnetically respond with a force of magnetic attraction or magnetic repulsion to the large magnetic fields produced in MRI systems, because such forces could cause movement of the patient's head and/or dislodgement of the cervical fixation device. Titanium, aluminum and non-ferromagnetic stainless steel all meet this requirement.
A fourth concern with regard to cervical fixation device components is associated with the peace of mind of the patient. Both the MRI procedure and the cervical fixation device are typically strange, new procedures for a patient who has recently been through a traumatic injury. As much as possible should be done to avoid instilling fear or panic in the patient during the MRI and cervical fixation procedures.
One reported way in which patient fear has escalated is associated with a "heating" sensation felt at the skull pin contact locations by some patients during some MRI procedures. There has generally been no evidence of redness or swelling of the scalp surrounding the skull pins in patients complaining of the "heating" sensation. At least one reported test indicates that the temperature increase of the skull pins during MRI is 1.5.degree. C. or less, even with SAR's which exceed by as much as four times the recommended guidelines. This magnitude of temperature change is considered to be relatively minor from a physiological standpoint. Factors reported as effecting the amount of heating that occurs during MRI include the geometry of the object, the distance of the object from the transmitting RF coil, the amount of RF power transmitted and other aspects of the procedure.
With test results showing such a low temperature increase in the skull pins, it has been hypothesized that the "heating" sensation felt by some patients was instead vibration of the skull pins created by the MRI, particularly when MTC was used. The "heating" sensation was deemed likely to occur when the frequency or degree of vibration is at a certain level that stimulates peripheral nerve receptors located in the subcutaneous region that detects sensations of pain and temperature changes. It was recommended that MTC not be used in conjunction with cervical fixation devices. (See Shellock, "MRI compatability of Ace Cervical Fixation Devices", S.M.R.T. Newsletter 1995, pp. 4-7).